There are many sources of vibration in rotating machinery. For example, in flight, vibrations are transmitted to a helicopter fuselage from the rotor blades. While these may have many vibrational modes, it has been recognized that the dominant vibration is a function of the number of blades and their rotational speed. This is specifically mentioned in U.S. Pat. No. 3,477,665 ("LeGrand") at col. 1, line 35 et seq.:
"Due chiefly to the aerodynamic asymmetries occurring on the revolving rotor blades of rotary-wing aircraft, the rotors are subjected to alternating loads at frequencies which are multiples of the rotor speed and the number of blades. These loads are transmitted to the fuselage and cause it to vibrate at the same frequencies."
It is also discussed in U.S. Pat. No. 3,836,098 ("Miyashita") at col. 1, line 9 et seq.:
"In general, rotorcraft such as helicopters are subjected to large vibrations during flight as compared with fixed-wing aircraft, and this is attributable mainly to the forces and moment exciting forces from a rotor. Namely, with .OMEGA. representing the rate of rotation of the rotor and n representing the number of blades, there are generated an exciting force having a frequency of n.OMEGA. (hereinafter referred to as the n.OMEGA. vibration) and an exciting force have a frequency of an integer multiple of the rate of rotation (hereinafter referred to as i.OMEGA. vibration). The former is unavoidable with a rotor having n blades, . . ."
When transmitted to the fuselage, such vibrations are both annoying to the air crew and passengers, and contribute to their fatigue.
In an attempt to reduce, if not completely eliminate, such vibrations, it has been proposed to create an opposing vibrational waveform of like amplitude and frequency, but 180.degree. out-of-phase with respect to the disturbance vibration. The thought here is that the created and disturbance vibrations, when superimposed, will oppose and substantially cancel one another. While this is theoretically possible, it must be remembered that the parameters of the rotor-preduced vibrations are functions of many other factors, such as the load carried by the helicopter, the attitude of the helicopter due to maneuvering, its speed, etc. Hence, the rotor-produced vibration is subject to continuous change for various reasons, some controllable and others not.
Since the rotor-induced vibrations are usually centered at a substantially constant frequency, and only deviate therefrom for short duration transients, a passive resonant-type vibration "absorber" is frequently utilized to generate opposing vibratory forces on the helicopter structure. Such an "absorber", often called a "tuned damper", is a single degree-of-freedom mass-spring system arranged to vibrate at its resonant frequency in response to the expected vibrations of the structure to which it is attached. When the structure causes the "absorber" to vibrate at its natural frequency, the reaction force exerted by the "absorber" on the structure will be out-of-phase with the vibratory displacement of the structure, but will be in-phase with the vibratory velocity of the structure. Hence, it will appear as "damping" or energy "absorption", and will have the effect of reducing the amplitude of the disturbance vibration at the mounting point. However, the amplitude of the opposing vibration will utilize the resonance phenomena only in the immediate vicinity of the natural frequency. This technique, and its limitations, are specifically referred to in U.S. Pat. No. 4,483,425 ("Newman") at col. 1, line 50 et seq.:
"Another related method of vibration control is passive vibration compensation, which uses inertial compensation through a resonant spring-countermass combination. This method is reasonably effective if the inertial force imbalance to be compensated is primarily sinusoidal at a single constant frequency. The spring-mass combination can be tuned to this frequency so that it responds to vibrations by oscillating to help cancel the vibrations. However, the effectiveness of this approach is limited because compensation only occurs at the single selected frequency, the amount of compensation depends upon the characteristics of the mechanical connection between the machine and its environment, and performance may seriously degrade with time or external influences."
(Emphasis added.)
In an attempt to follow frequency variations in the disturbance vibration, some mass-spring "absorbers" have been designed to permit continuous adjustment of either the effective mass or the effective spring rate, with some criteria for recognizing the optimum "tuning". In one such device, as shown in U.S. Pat. No. 4,365,770 ("Mard" et al.), the preload on a cam-operated spring is adjusted by a hydraulic servo to "tune" the effective spring rate of a mass-spring "absorber" so that its natural frequency will substantially equal the frequency of the disturbance vibration. In this case, the spring preload position is calibrated to allow it to be set as a function of the measured vibration frequency. In another application of this technique, the radius of a pendulous mass is adjusted, by means of an electric motor and screw, to change the effective mass restrained by a fixed spring rate. Optimum "tuning" is sensed by comparing the relative phase relationship of an accelerometer measuring structural vibration to another accelerometer on the moving mass. Yet another "tuning" technique uses a motor-driven adjustable-ratio four-bar linkage to alter the relative motion of a vibrating mass. By affording the capability of varying either the effective mass or the effective spring rate, each of these arrangements overcomes the normal sharply-defined resonance of a "tuned" absorber having a fixed mass and a fixed spring rate, but does so at the expense of mechanical complexity.